Diffusion of a P type impurity into a predetermined region of a semiconductor material is sometimes used in the process of manufacturing a semiconductor device. FIG. 5 is a view showing a semiconductor laser element in which a current constriction structure and an optical waveguide structure are formed using the conventional method of diffusing a P type impurity. In FIG. 5, reference numeral 1 designates an n type GaAs substrate on which an n type GaAs lower cladding layer 2, a GaAs active layer 3, a p type AlGaAs upper cladding layer 4, an n type AlGaAs layer 5 and a p type GaAs contact layer 6 are sequentially disposed. Reference numeral 9 designates a Zn diffusion region formed from the contact layer 6 to reach the active layer 3. An n side electrode 16 and a p side electrode 17 are formed on a back surface of the substrate 1 and on the contact layer 6, respectively.
Next, a method for forming the Zn diffusion region 9 in the process of manufacturing the semiconductor laser element will be described in reference to FIGS. 3(a)-3(c). The n type AlGaAs lower cladding layer 2 having a thickness of approximately 1 micron, the GaAs active layer 3 having a thickness of approximately 0.1 micron, the p type AlGaAs upper cladding layer 4 having a thickness of approximately 1 micron, the n type AlGaAs layer 5 having a thickness of approximately 0.5 micron and the p type GaAs contact layer 6 having a thickness of approximately 1 micron are sequentially formed on the n type GaAs substrate 1 by epitaxial growth, whereby the structure shown in FIG. 3(a) is obtained. As the growth method, for example a metal organic chemical vapor deposition (MOCVD) method is used. Thereafter, an SiN film 7 serving as a selective diffusion mask is grown on the contact layer 6 and a striped opening pattern having a width w1 is formed at a region where the active region is to be formed as shown in FIG. 3(b). A film 8 serving as a diffusion source in which ZnO and SiO.sub.2 are mixed in the ratio of 9:1 is formed on the SiN film 7 having the opening pattern and on the contact layer 6 exposed in the opening as shown in FIG. 3(b). Then, a SiO.sub.2 film 15 serving as a surface protecting film at the time of annealing is formed on the ZnO/SiO.sub.2 mixed film 8 and then annealing is performed at a diffusion temperature of 650.degree. C. for 1 to 2 hours, whereby Zn is diffused into a wafer from the ZnO/SiO.sub.2 mixed film 8 and a Zn diffusion region 9 is formed as shown in FIG. 3(c). Actually, since the diffusion speeds of Zn in GaAs and in AlGaAs are different, the Zn diffusion region 9 spreads differently in the width direction in each layer. However, the cross-sectional boundary of the Zn diffusion region 9 is shown in the form of a partial ellipse in FIG. 3(c) for the sake of simplicity. After the diffusion process, the SiO.sub.2 film 15, the ZnO/SiO.sub.2 film 8 and the SiN film 7 are removed and then the n side electrode 16 and the p side electrode 17 are formed. Thereafter, the wafer is divided into chips, whereby the laser element shown in FIG. 5 is completed.
The above semiconductor laser element is called a diffusion stripe (DS) type laser. Since the n type AlGaAs layer 5 serves as a current block layer on both sides of the current passage formed by the Zn diffusion region, a current constriction structure is obtained. In addition, in the active layer 3 the refractive index of the region 9 where Zn is diffused is higher than that of the region on both sides thereof where Zn is not diffused, so that light confinement structure in the horizontal direction is achieved. Light confinement in the vertical direction is achieved by the double-heterojunction structure so that a waveguide is formed in the active layer.
According to the DS type semiconductor laser shown in FIG. 5, the width of the active region is determined by the width of the Zn diffusion region 9. The width of the active region of the semiconductor laser has to be set at approximately 2 microns in order to obtain oscillation in a fundamental transverse mode. However, since the same diffusion occurs not only in the depth direction but also in the width direction in the conventional method for diffusing the P type impurity, it is difficult to control the width of the active region. In order to form the Zn diffusion region which penetrates the active layer 3 from the contact layer 6 and reaches the lower cladding layer 2 as shown in FIG. 3(c), a diffusion depth of approximately 3 microns is necessary. The same diffusion in the depth direction occurs also in the width direction as shown by w2 in FIG. 3(c), so that the width w3 of the Zn diffusion region 9 in the active layer 3 is larger than the width w1 of the opening of the SiN film 7 shown in FIG. 3(b). Therefore, the width w1 of the opening of the SiN film 7 must be smaller than a desired width of the active region, for example, at approximately 1 micron. However, since the width w3 of the Zn diffusion region 9 in the active layer 3 depends on the diffusion depth, it is very difficult to precisely control it.
Meanwhile, a precise, narrow P type region may be formed by implantation of P type ions. In this method, a P type impurity is implanted into a semiconductor by irradiating an upper part of the wafer with an anisotropic ion beam using a dielectric film as a mask. According to this method, it is possible to form a narrow P type region because the impurity does not spread in the horizontal direction. However, since the impurity concentration formed by ion implantation is on the order of 10.sup.18 cm.sup.-3 at the most, it is not applicable to the above laser structure which requires an impurity concentration of 10.sup.19 cm.sup.-3 or more.
Since the prior art method for diffusing a P type impurity by thermal diffusion is as described above, it is difficult to control diffusion to a narrow region because of diffusion in the horizontal direction.
Further, although ion implantation forms a narrow P type region, sufficient impurity concentration can not be obtained in that method.